Base station and call search method used for the same

ABSTRACT

In a base station providing a large-distance service area (large area) such as a mobile communication system WLL of the CDMA method, the time required for performing call search (path search) is reduced so as to reduce the call connection time. When performing a path search in the base station ( 8 ), a predetermined offset value from a reference time is set so that path search is performed only in a doughnut-shaped service area ( 30 ). Thus, there is no need of performing path search in the vicinity of the base station ( 8 ) and search is performed only in the doughnut-shaped service area ( 30 ). Thus, it is possible to reduce the path search time and reduce the call connection time.

TECHNICAL FIELD

The present invention relates to a base station and a call search methodused in the same, and in particular, to a call search method used in abase-station wireless apparatus provided with a long-distance servicearea (large area) in a mobile communication system typified by WLL(Wireless Local Loop) employing the CDMA (Code Division Multiple Access)system.

BACKGROUND ART

There is a mobile communication system employing the CDMA system. Inthis kind of communication system, as shown in FIG. 1, a base-stationwireless apparatus (hereinafter simply referred to as a base station) 8provides CDMA communication service, which ideally has a service areaconcentrically extending around the centrally located base station 8 asshown in FIG. 1( a) by ordinary. Since this is always the sameregardless of the size of the service area, a long-distance service areaalso concentrically extends as shown in FIG. 1( b) in the same manner as(a).

As shown in FIG. 2, since the base station 8 always searches the servicearea for a calling party (a person who telephones by radio) as ripplingout from the center to the circumference that is the end of the servicearea, the base station 8 takes a longer time (call connection time) tosearch for the calling party as the service area that covers the circlebecomes large.

PROBLEMS TO BE SOLVED BY THE INVENTION

There is a mobile communication system typified by WLL provided with along-distance service area (large area) employing the CDMA system basedon IS-95 Standard by the Telecommunication Industry Association. Asshown in FIG. 3, such WLL communication system often supports the areas21 on the circumference 20 more than a few score kilometers distant fromthe centrally located base station 8 instead of covering the area in thevicinity of the base station 8 and designing plural areas to beoverlapped as in the case of performing soft hand-off since nocommunication terminal is movable.

Also in this case, as explained with FIG. 2, when constantly performingthe search for the calling party as rippling out from the center to thecircumference 20, it takes longer time to conduct call search (referredto as path search), which giving rise to a problem of a longer callconnection time. Moreover, the terminal has to continuously transmitcall signals during the longer call connection time, which becomesinterference with the other users.

It is therefore an object of the present invention to provide a callsearch (path search) method used in a base station to reduce the callconnection time in a long-distance service area (large area) such asWLL.

It is another object of the present invention to provide a call search(path search) method capable of reducing interference between terminals(between users) by shortening the path search time.

DISCLOSURE OF THE INVENTION

According to the present invention, there is provided a base station forsearching for a call from a terminal in a service area, comprising asearch means for setting a predetermined offset value based on a basetime to perform the search.

According to the present invention, there is provided a call searchmethod used in a base station that searches for a call from a terminalin a service area, comprising a search step for setting a predeterminedoffset value based on a base time to perform the search.

The following are an explanation of an operation of the presentinvention. As described above, generally, since a WLL system does nothave the area in the vicinity of the base station as a service area butoften supports areas on a circumference more than a few score kilometersdistant from the base station as a center, a service area is defined ina doughnut shape for performing optimization so as to reduce a pathsearch time and a call connection time. To achieve this, when the basestation performs the path search, a predetermined offset value based ona base time is set for the path search. By this means, the path searchis not performed over the area in the vicinity of the base station butperformed only within the doughnut-shaped service area, consequentlyreducing the path search time.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing an example of a conventional service area.

FIG. 2 is a diagram for explaining a search method in a base station.

FIG. 3 is a diagram showing an example of a long-distance service area.

FIG. 4 is a schematic illustration showing a service area according toan embodiment of the present invention.

FIG. 5 is a block diagram showing a base station according to theembodiment of the present invention.

FIG. 6 is a diagram showing an example of a timing of a search by thebase station and a timing of a call by a mobile terminal.

FIG. 7 is a flowchart showing an operation according to the embodimentof the present invention.

Incidentally, the reference numeral 1 denotes an antenna. The referencevalue 2 indicates an antenna-shared device. The reference numeral 3shows a receiver. The reference numeral 4 denotes a search section. Thereference numeral 5 indicates a demodulator. The reference numeral 7shows a transmitter. The reference numeral 8 denotes a base station. Thereference numeral 30 indicates a doughnut-shaped service area.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, an explanation will be given of an embodiment of thepresent invention in reference to Figures. FIG. 4 is a diagram showing arelationship between a base station 8 and its service area 30 accordingto the embodiment of the present invention. According to the presentinvention, the service area 30 is optimized in the doughnut shape asshown in FIG. 4 instead of providing a conventional concentric-circleservice area (cf. FIG. 1) on the basis of a fact that the WLL systemdoes not have the service area in the vicinity of the base station 8 andnot design areas to be overlapped as in the case of performing softhand-off since no terminal is movable but often supports areas on acircumference more than a few score kilometers distant from thecentrally located base station 8 as described hereinbefore with FIG. 3,thereby reducing the path search time and the call connection time.

For example, when the radius of the small area shown in FIG. 1( a) andthe radius of the large area shown in FIG. 1( b) are determined as “1”and “10”, respectively, and when the inner radius R₁ and the outerradius R₂ of the doughnut-shaped area in FIG. 4 are determined as “9”and “10”, respectively, the time required for the path search becomes“1” in the case of FIG. 1( a), “10” in the case of FIG. 1( b), and “1”in the case of FIG. 4. While the time required for the path search inthe case where the large area is designed using the conventional methodis ten times as long as that in the case of the small area, the pathsearch time in the case of the optimized doughnut-shaped search areabecomes equal to the path search time in the case of the small area,from which it is apparent that the call connection time is drasticallyreduced.

FIG. 5 is a block diagram showing the base station 8 using the CDMAsystem according to the embodiment of the present invention. In FIG. 5(a), the base station 8 comprises an antenna 1 for relaying signalstransmitted and received between a calling party and the base station 8,an antenna-shared device 2 having a filter function for separating thesignals relayed via the antenna 1 into receiving signals andtransmitting signals, a receiver 3 for amplifying a received signal toremove interfering waves in the vicinity of the received signal, asearch section 4 having parameters to change a path search area forfinding an effective signal in the received signal from which noise hasbeen removed, and a demodulator 5 for demodulating the effective signalfound by the search section 4 to transmit it to an upper device notshown. Further, the base station 8 comprises a modulator 6 formodulating a signal from the upper device and a transmitter 7 forconverting the modulated signal into a transmitting signal to transmitit to the antenna 1 via the antenna-shared device 2.

FIG. 5( b) is a block diagram showing a detail of the search section 4in FIG. 5( a). The search section 4 comprises a radius data section 10for inputting radius data on an inner radius R₁ and an outer radius R₂of the doughnut-shaped service area 30 shown in FIG. 4, a chipconverting section 11 for converting the radius data into chip-unitdata, and a path search section 12 for determining a timing of pathsearch on the basis of a chip offset (an offset value for determining adelay of a start time of path search from a base time) calculated by thechip converting section 11 to perform the path search.

The “chip” is a code for spreading a data symbol used in the CDMAsystem, and 1 chip=1/1.2288 MHz=0.814 μ s since an IS-95 system has a1.2288 MHz band.

In the following, before giving an explanation of an operation of theembodiment, an explanation will be given of an operation leading up tosearching the small area in FIG. 1( a) for a calling party. First,radius data (inner radius and outer radius) for identifying the servicearea is input into the radius data section 10. In the case of a normalsmall area as shown in FIG. 1( a), the inner radius is “0”. The chipconverting section 11 converts the input radius data into chip-unitdata, and subsequently, the path search section 12 searches an effectivepath area during the period from a base time (which is a base time onthe basis of GPS) until the time delayed by the offset on the basis ofthe chip offset.

Hereat, since the IS-95 system has a 1.2288 MHz band, 1 chip=0.814 μs,and a search moving distance Lc per 1 chip becomes as follows.Lc=0.814μs×C=244m

Hereat, C is a velocity of light, and obtained by 3×(10 raised to theeighth power) m/s.

For example, in the normal area with radius 10 Km, the inner radius is 0Km, the outer radius is 10 Km, and the search area (width) becomes asfollows.10,000/244=41 chipsAccordingly, when searching the area with radius 10 Km, it is necessaryto perform the search in the time range of the “base time+0 chip” to the“base time+41 chips”.

On the other hand, the base station constantly performs the search inthe order of {circle around (1)}→{circle around (2)}→{circle around(3)}→{circle around (1)}→ . . . as shown in FIG. 6( a) regardless of acall from a terminal. The time required for the search depends on asearch width and the number of paths (the number of antennas), and thetime required for the search is roughly calculated as follows in thecase of the IS-95 system.Search time=(search width×2)×the number of antennas×(ACC/Rakewidth)×Twalsh  (1)

Hereat, the number of antennas is “3” since 3-sector configuration isemployed in general, the ACC is the number of symbols to be accumulatedand determined to be “2”, and the Rake width is the number of elementsused for the search and determined to be “25”. Moreover, the Twalsh is atime per 1 symbol and is 208 μs. The above-described respective valuesare fixed values.

If the aforementioned values are substituted into Equation (1), it wouldbe found that the search of the area with a radius of 10 Km requires asearch time of up to 4.1 ms.

As shown in FIG. 6( b), when a search and a call are performed in goodtiming, the time required for the search is reduced enough to be lessthan 4.1 μs since a point {circle around (2)} where a mobile terminalactually transmits a call is in the direction in which the search of thebase station 8 proceeds from the point {circle around (1)} where thebase station 8 is currently performing the search. On the other hand, asshown in FIG. 6( c), when a search and a call are performed in badtiming, the base station 8 takes 4.1 ms at the maximum to find out themobile terminal since a point {circle around (2)} where a mobileterminal actually transmits a call is distant from the area {circlearound (1)} where the base station 8 performs the search.

The following is detail calculation of time required for searching for acalling party in the large area (with radius 100 Km) in FIG. 1( b). Whencalculating a chip offset in the same manner as the above method, thebase station 8 performs the search during the time period from the “basetime+0 chip” to the “base time+410 chips”, namely, the base station 8takes 41 ms to perform the path search.

The following is detail calculation of time required for searching for acalling party in the doughnut-shaped area 30 of the present inventionshown in FIG. 4. The radius data section 10 in the base station 8 inputstherein the inner radius R₁=90 Km and the outer radius R₂=100 Km. Thechip converting section 11 divides the inner radius and the outer radiusby the moving distance Lc per 1 chip, respectively, and then the numberof chips “369” corresponding to the inner radius 90 Km and the number ofchips “410” corresponding to the outer radius 100 Km are obtained. Thenumber of chips “369” corresponding to the inner radius is used as anoffset value, and accordingly, the path search section 12 needs tosearch only the area corresponding to the 41 chips from the “basetime+369 chips” to the “base time+410 chips”, thereby realizing the pathsearch in a few milliseconds in the same manner as searching the areawith radius 10 Km as described above (FIG. 1( a)).

FIG. 7 is a flowchart showing an operation according to theabove-described embodiment of the present invention. In reference toFIG. 7, first, the radius data section 10 in the base station 8 inputstherein radius data on the inner radius R₁ and the outer radius R₂ (StepS1). The inner radius R₁ serves as an offset value that determines astart timing of a search time, and the outer radius R₂ serves as a valuethat determines a finish time for searching the area. Subsequently, thechip converting section 11 performs calculations as R₁/Lc and R₂/Lc toperform chip conversion (Step S2). Subsequently, the path search section12 performs path search during the time period as follows (Step S3).“Base time+R₁/Lc”˜“Base time+R₂/Lc”

The call connection time is actually influenced by a transmission timeinterval (referred to as ACCESS PROBE SEQUENCE in the case of an IS-95Asystem) of a call signal from a mobile terminal. For example, when themobile terminal repeats the sequence of a transmission of a call signalin 20 ms and a pause in 1.6 s, the base station has no chance to receivethe next call signal from the mobile terminal until 1.36 s passes unlessthe base station finishes the search within the transmitting time of thefirst 20 ms.

This call sequence of the mobile terminal is performed based ondirections in a message informed from the base station, the directionsbeing regulated in Chapter 6 of the IS-95A in detail. Namely, in thiscase, when the speed at which the base station searches the whole areaexceeds 20 ms, there is reduced the likelihood that the base station canreceive the first call signal, which leads to a longer call connectiontime. Therefore, according to the present invention, by using adoughnut-shaped service area as a large area (long-distance servicearea), it becomes possible to restrain the increase of theabove-described call connection time.

INDUSTRIAL APPLICABILITY

As described hereinbefore, according to the present invention, anoptimum area search method, or a doughnut-shaped service area method isemployed in the CDMA system having a large service area not only toreduce the call connection time and to provide improved services forusers but also to reduce the generation of call signals, which have tobe constantly transmitted until a connection to a network isestablished, for reducing interference with the other users.

1. A base station for performing a search for a call from a terminal ina service area having a predefined size, said base station comprising: asearch section, said search section including: a data section forinputting first and second distance data of the service area; and a pathsearch section for determining a timing of a path search on the basis ofa chip offset, said chip offset being a predetermined offset value basedon a base time to perform the search in an area less than said servicearea and between said first and second distances, wherein the searchsection determines a time delayed from the base time by thepredetermined offset value as a search start time, wherein the searchsection performs the search during the period from the search start timeto a predetermined search finish time, and wherein the search sectionfurther includes a chip converting section for converting, in responseto inputs of parameters that are to serve as the offset value and thesearch finish time, the parameters into the offset value and the searchfinish time.
 2. A base station as claimed in claim 1, wherein theparameters are used for determining an area of the service area.
 3. Thebase station as claimed in claim 1, wherein the predetermined offsetvalue is used to determine a doughnut shaped service area.
 4. A callsearch method used in a base station for performing a search for a callfrom a terminal in a service area having a predefined size, said methodcomprising: inputting distance data for identifying the service areainto a data section; and a search step for setting a predeterminedoffset value based on a base time to perform the search in an area lessthan said service area and set by said distance data, wherein, in thesearch step, a time delayed from the base time by the predeterminedoffset value is determined as a search start time, wherein, in thesearch step, the search is performed during the period from the searchstart time to a predetermined search finish time, and wherein the searchstep further including a step for converting, in response to inputs ofparameters that are to serve as the offset value and the search finishtime, the parameters into the offset value and the search finish time.5. A call search method as claimed in claim 4, wherein the parametersare used for determining an area of the service area.
 6. The call searchmethod as claimed in claim 4, wherein the predetermined offset value isused to determine a doughnut shaped service area.
 7. A base station forperforming a search for a call from a terminal in a service area havinga predefined size, said base station comprising: a search section, saidsearch section including: a data section for inputting first and seconddistance data of the service area; a chip converting section forconverting the distance data into chip-unit data; and a path searchsection for determining a timing of path search on the basis of a chipoffset, said chip offset being a predetermined offset value fordetermining a delay of a start time of a path search from a base timethat is calculate by the chip converting section to perform the pathsearch.
 8. The base station as claimed in claim 7, wherein the distancedata is radius data.
 9. The base station as claimed in claim 8, whereinthe predetermined offset value is used to determine a doughnut shapedservice area.